1. Field of the Invention
The present invention relates to a thermal transfer sheet for use in sublimation thermal transfer recording, and more particularly, to a thermal transfer sheet that yields a picture image having an improved light-resistant colorfastness.
2. Discussion of the Related Art
A sublimation thermal transfer recording method has been known in the art as a simple method for producing full-color picture images. In the sublimation thermal transfer recording method, a sublimating dye of yellow, cyan, or magenta and an appropriate binder resin are coated on one face of a base film, such as a polyester film, to form a dye layer with the corresponding hue. The thus produced three thermal transfer sheets bearing respective colors are used to produce full color images on an image-receiving sheet. A chromophil thermal-transfer image-receiving sheet is alternately overlaid with the thermal transfer sheets bearing the respective three colors (and black, if necessary), and each dye on the respective thermal transfer sheet is sublimated and transferred onto a dye-receiving layer of the image-receiving sheet by a thermal head printer, thereby enabling regeneration of a full color picture image from an original.
Although the dyes for use in the thermal transfer sheets for respective colors should be selected from yellow, magenta, and cyan dyes having ideal huse--the dyes used in other printing methods such as offset printing, for example--in order to precisely reproduce the colors in the original picture, it is practically difficult to generate an ideal hue using merely one kind of dye. Conventionally, a plurality of dyes are blended for each color to produce acceptable hues.
Among the three thermal transfer sheets bearing respective colors, the cyan thermal transfer sheet is particularly difficult to regenerate an ideal cyan color if only one kind of cyan dye is used. A nearly ideal cyan is obtained by blending two or more kinds of cyan dyes. However, when the picture is formed by using such a conventional cyan thermal transfer sheet, the quality of the resulting full-color picture image degrades as time elapses; i.e., a light resistance is not sufficient. It is considered that this degradation occurs because of photo decomposition or photodegradation of the constituent dyes, which in turn occurs because the cyan dyes transferred from such a cyan thermal transfer sheet to the dye receiving layer exert catalytic effects on each other in the dye receiving layer under external light irradiation. When the cyan color fades or changes in the full-color picture image, the overall quality of the full-color picture image suffers considerably.